DACs are indispensable in handling signals where a digital word/signal is to be converted into an analog form. DACs are an essential tool in handling communication signals in information technology including television, sound and movies, as well as handling signals in medical and industrial control loops. DACs are available in several forms, and a common and popular configuration uses a resistor string driven by voltage sources. In U.S. Pat. No. 6,621,440 B2, at least two resistor ladders arranged in a cyclic string are used with two banks of switches. An MSB string is used and connected to complete the function as a DAC and is coupled at two nodes to voltage reference points VREF+ and VREF−, through a virtual short. A LSB (least significant bit) string is coupled across an adjacent node on the MSB (most significant bit) string. Two analog output signals are generated from the ends of the connecting resistors. The output analog signals might be produced from further first and second resistance ladders. The main resistance ladders are driven by first and second reference voltages. U.S. Pat. No. 5,619,203 titled Current Source Driver Converter, issued to George F Gross, Jr et al, on Apr. 8, 1997, teaches a DAC having a resistor string coupled to a current source. U.S. Pat. No. 5,604,501 titled “Digital to Analog Converter with reduced number of resistors”, issued to McPartland on Feb. 18, 1997, teaches a DAC using two voltage sources and a resistor string having resistor-potential junctions and taps at resistor junctions. U.S. Pat. No. 5,283,580 titled “Current/Resistor Digital to Analog Converter having enhanced integral non-linearity and method of operation,” issued to Todd L Brooks on Feb. 1, 1994, teaches a DAC using series connected resistors using current sources which can be switched to either a first or a second node. The resistors in the Brooks patent are connected between a reference voltage terminal and a third node where an analog output signal is developed.
For design of high precision, low power, and area efficient DACs, resistor string DAC architectures are of prime importance. For high resolution segmented string DACs, the static and dynamic performance is limited by the area of the DAC Core, resistor matching the circuit non-linearity, and first and second order effects like the coupling switch resistance and sub-threshold/leakage current. There are other arrangements in prior art, in which multitude of segmentation and coupling schemes are introduced, using either linear resistance strings or resistance strings driven by voltages. There is generally a loss of implementation area in the prior art arrangements.
Therefore, there is a need for DAC architecture which will be at least as area-efficient as (or better than) the segmented string architecture, while improving some of the key performance indices such as linearity and speed of operation. It is also noted that prior art arrangements offer a limited scope of dynamic element matching because of lack of redundancy caused by the use of conventional resistor string arrangements.